1. Statement of the Technical Field
The inventive arrangements relate to antennas, and more particularly to broadband antennas for portable devices
2. Description of the Related Art
Short flexible monopole antennas are commonly used for portable communication devices. For example, Harris Corporation of Melbourne, Fla. offers a broadband blade antenna (Model No. 12011-2710-01) which operates over a 30 to 512 MHz frequency range and is 13 inches long, and a unity gain rubber-duck antenna (Model 12102-2700-01) which operates over (30-870 MHz), which is only 9 inches long. These antennas are compact in size to satisfy customer demands. However, antenna size also has an effect on antenna performance, and it is common for smaller antennas to sacrifice performance to facilitate smaller physical size.
Antenna matching networks are often required in order to facilitate use of a single antenna over a broad range of frequencies. These matching networks perform an impedance transformation function. At each frequency of operation, the matching network transforms an impedance of the antenna to approximately match the input or output impedance of the communication device. This impedance matching function facilitates efficient power transfer between the antenna and the communication device. Matching networks can be formed from lumped elements, RF transmission line sections, or a combination of the two.
Some short flexible monopole antenna designs include matching networks integrated directly into the antenna assembly. Usually, the matching network is integrated into the base of the antenna, near where it connects to the portable communication device. Typically the matching network extends from an output port or antenna connector of the portable communication device, to a base end of the monopole antenna radiating element that is nearest to the radio. Consequently, the RF feed gap of the monopole radiating element may be spaced somewhat away from the chassis of the portable radio in order to accommodate the physical length of the matching network. From the foregoing, it can be understood that a first portion of the overall length of the antenna can be allocated to the matching network and a second portion of the overall length can be allocated to the radiating element. Consequently, the overall length of the antenna assembly is directly affected by the size and arrangement of the matching network. The matching network can be relatively large, particularly when an antenna is designed for handling relatively high power levels. For a fixed length matching network, the relative or percentage portion of the overall antenna length devoted to the matching network actually increases as the radiating element length is decreased.